Method and Device for Measuring of Oedema

ABSTRACT

The present invention concerns a method and a device for measurement and assessment of oedema, especially with regard to a combination of the two types of oedema called pitting oedema and brawny oedema. The method comprises the steps of reading the frequency of a freely and in resonance oscillating sensor, the application of the in resonance oscillating sensor against an oedema to a certain depth of depression and the continuous reading of the force of application and the resonance frequency of the in resonance oscillating sensor. The device comprises a measuring head that is adjustably supported by a stand and exhibits partly a freely and in resonance oscillating sensor ( 6 ) for making a resonance system together with the oedema when applied to it, partly a means of reading the current resonance frequency of the sensor, partly means ( 5 ) for measuring the force with which the sensor is applied to the oedema. Information is hereby obtained concerning the degree of stiffness as a measure of brawny oedema through read frequency shift and the degree of displaced fluid obtained as a measure of pitting oedema through measurement of decay of force.

FIELD OF THE INVENTION

The present invention concerns a method and a device for measurement and assessment of oedema, especially with regard to a combination of the two types of oedema called pitting oedema and lymphatic oedema.

BACKGROUND OF INVENTION/KNOWN TECHNOLOGY

Oedema—“swelling”—is a classic symptom of illness. The presence of oedema can be used as in indicator of various kinds of illness, such as heart and kidney disease, thrombosis, burns and lymph circulation disorder. For patients, oedema involves a number of inconveniences/problems, such as pain, change in centre of gravity leading to back problems and a general impairment to quality of life. Consequently, there is a need for measuring equipment to assess an oedema and measure how oedema is reduced by various treatments in hospital. Oedema can generally be divided in to “pitting oedema” and “brawny oedema”. Brawny oedema arises when fluid that causes the swelling remains inside the cells and not between them, as is the case with pitting oedema. Brawny oedema can also arise when the fluid causing the swelling is located between the cells but has coagulated due to the precipitation of fibrinogen. Often, pitting oedema and brawny oedema arise simultaneously in the same patient and it is known in literature that pitting oedema often turns into brawny oedema after a time.

Problems within Known Technology

Current methods of measuring oedema are principally designed to indirectly measure the volume of an oedematous (swollen) extremity and compare it with the volume of the corresponding normal extremity (volumetry). For example, such measurements for arm oedema are taken by placing the swollen arm into a cylinder full of water and then measuring the weight of the overflowing water. The same procedure is repeated with the normal arm and the different in weight of the overflowing water gives a measure of the size of the oedema. Understandably, this method is very imprecise and small differences in the size of an oedema cannot be measured. Neither can the distribution of different types of oedema nor their location be determined with volumetry.

In recent years there have been a number of identometers/tonometers presented in the literature. These measure the occurrence of pitting oedema locally by e.g. depressing the skin at the site of the swelling and holding a depth of depression for a certain time of measurement, e.g. 20 seconds. The force required to maintain the depth of depression is registered and will become lower during the time of measurement depending more or less on how much of the free flowing oedema fluid that is displaced in the tissue under pressure.

One example of the tonometric method is the impression method that presses a circular measuring plate (e.g. D=15 mm) into the skin, normally 4 mm, and measures the force required during a period of 20 seconds. Lindahl and Omata (1993) made a comparison between the impression method and a tactile sensor with regard to the ability of the two instruments to detect pitting oedema.

The tonometric methods are limited in that they cannot measure brawny oedema as they assume the fluid in the oedema is free flowing.

OBJECT OF THE INVENTION

The object of the present invention is to relieve or overcome the aforesaid problems and achieve a method and a device for measurement of oedema with regard to a combination of pitting oedema and brawny oedema.

BRIEF DESCRIPTION OF THE INVENTION

This objective is achieved by means of a method and a device respectively according to the attached independent claims.

The distinctive features and benefits of the invention given in these claims and other distinctive features and benefits will be obvious to a skilled person taking notice of the following detailed description of the embodiments of the invention that do not limit the scope of protection of the invention.

The embodiments are described with reference to the attached drawing, in which identical or similar parts have been given the same reference designations.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic view of an arrangement according to a first embodiment of the present invention.

FIG. 2 shows the measuring head according to FIG. 1 in larger scale and partial cross-section.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Generally, the arrangement for measurement of an oedema comprises a tactile sensor for measuring hardness/stiffness combined with the measurement of the reduction in the force required to depress tissue, e.g. skin or organ. This combination provides information on partly the degree of hardness by measurement with the tactile sensor, partly the amount of displaced fluid in the tissue by measurement of the decay of force.

With this arrangement, the decay of force in relation to the change in hardness can be used as a measure of the size of the oedema, where the size of the oedema comprises an evaluation of both the fluid content and brawn.

Unlike known methods, such an arrangement can:

-   -   Evaluate and locate local occurrences of brawny oedema     -   Evaluate the results of treatment where brawny oedema are         mobilised, e.g. through massage, and change to pitting oedema.     -   Evaluate the degree of pitting in relation to brawn

The embodiment according to FIG. 1 illustrates a measuring setup with a stable stand 2 exhibiting securable flexible jointed arms for positioning of a measuring head. The arms are attached with lockable joints 3. A stepping motor 1 is arranged at one free end of the stand and can by means of computer control, not shown, be made to move the measuring head by means of a shaft 7 against the skin of a patient where oedema is, e.g. a human leg 8. The measuring head comprises a resonance sensor 6, which can be of a piezoelectric material. The resonance sensor 6 is made to vibrate at a known resonance frequency using an electric feedback circuit, which feeds back the vibration via a relatively smaller piezoelectric element built into 6. The difference between the frequency of a freely oscillating sensor and the frequency when the sensor is pressed against tissue is called Δf and is a measure of the acoustic impedance of the tissue, i.e. the hardness or stiffness of the tissue.

There is also a force sensor 5. The force sensor 5 is connected to the resonance sensor 6 so that the force that arises through the application of the resonance sensor 6 against the skin 13 can be registered without the force sensor 5 interfering with the vibration of the sensor 6. This can be achieved by casting the sensor 6 into an elastic sleeve 10 made of for example crude rubber that allows the sensor to vibrate freely. The sleeve 10 can then be enclosed in a larger sleeve by means of spring washers 9, which allow the sleeve 10 to affect the force sensor 5 which thereby can measure the sensor's application force without affecting its vibration.

The stepping motor 1, the force sensor 5 and the resonance sensor 6 are all connected via interface electronics to a personal computer, not shown, which controls and measures via software. When the sensor package 5, 6 in FIGS. 1 and 2 reaches the surface of the skin 13 on the leg 8, the force sensor 5 indicates that the surface has been reached through deflection of the force sensor, e.g. 50 mN. The motor 1 is then instructed by the computer to press in the measuring plate 11 to a certain depth in the skin, e.g. 4 mm. The measuring plate 11 has a known application surface, e.g. a circular cross section 12 of 15 mm. This leads to the measuring plate 11 achieving a depression in the skin corresponding to a cylinder of diameter 15 mm and height 4 mm. The volume of depressed skin can then be calculated according to V=πh(D/2)²=π4(15/2)²=0,707 ml. By measuring the decay of the force over a period of time, e.g. 20 seconds, volume-related parameters can be calculated, e.g. as V(t)=(1−Fn(t))πh(D/2)² where Fn is normalised force, which provides information on the amount of free flowing fluid in the tissue that has been depressed.

To achieve a cylindrical depression, it is necessary for the measuring plate 11 to be parallel with the surface of the skin 13. This can be ensured with the stepping motor 1 and shaft 7 being fixably moveably arranged 4 on the stand, which allows the measuring plate to be adjusted so that it is parallel with the surface of the skin before application against it.

With this embodiment the hardness of the tissue can be measured with the resonance sensor at the same time as the volume of the free-flowing fluid can be estimated. Thus, a combined pitting and brawny oedema can be measured.

In an alternative embodiment, measuring can be done with the motor driven sensor package 1, 5, 6, 7 in a hand-held version without the fixed stand in FIG. 1. In this embodiment, the stand can be a counter hold arrangement, e.g. a rim, arranged on the motor sensor package and furnished with one or more supporting points arranged to support on the skin/tissue around, but at a distance from, the tip of the sensor so that a sustained depression safely can be achieved for at least 20 seconds without dislodging the sensor package. The distance from the tip of the sensor is adjusted so that the supporting points do not affect the tissue in which measurements are being done.

In a further embodiment, the measuring head 5, 6 can be arranged manually displaceable to the stand, so that it exhibits two mechanically limited positions. One starting position and one measuring position. When a measurement is to be taken, the measuring head is in its starting position and the stand is adjusted so that the application surface of the resonance sensor 6 is parallel to, and flushes with, the tissue to be measured. After that the measuring head is displaced manually to its active measuring position and measurement can start. Hereby a certain depth of depression is obtained without adjustable electric stepping motors. Mechanical application, however, does not offer the flexibility concerning the desired depth of depression that the adjustable stepping motor does.

When using the method according to the present invention simultaneous reading of frequency shift Δf and force F can be done, whereby a parameter Δf/F can be calculated that describes the consistency of the oedema or the relative relationship between pitting and brawn. This simultaneous reading can be done continuously and e.g. be plotted as Δf vs. F, whereby the incline can constitute a measure of the consistence of the oedema. 

1. Method for measurement of oedema, comprising the reading of the frequency of a freely and in resonance oscillating sensor, that the in resonance oscillating sensor is being applied against an oedema to a certain depth of depression, that the force of application and the resonance frequency of the in resonance oscillating sensor is continuously read, whereby information concerning the degree of hardness is obtained as a measure of brawny oedema through read frequency shift and the degree of displaced fluid as a measure of pitting oedema through measurement of decay of force.
 2. Method according to claim 1 characterised in the simultaneous reading of frequency shift Δf and force F and that a parameter Δf/F is being calculated describing the consistency of the oedema or the relative relationship between pitting and brawn.
 3. Method according to claim 2, characterised in that the simultaneous reading is done continuously and is plotted as Δf vs. F, whereby the inclination of the plotted curve constitutes a measure of the consistency of the oedema.
 4. Device for measurement of oedema, comprising a measuring head adjustably supported by a stand, characterised in that the measuring head exhibits partly a freely and in resonance oscillating sensor (6) for making a resonance system together with the oedema when applied to it, partly a means of reading the current resonance frequency of the sensor, partly means (5) for measuring the force with which the sensor is applied to the oedema, whereby information concerning the degree of hardness is obtained as a measure of brawny oedema through read frequency shift and information concerning the degree of displaced fluid as a measure of pitting oedema through measurement of decay of force.
 5. Device according to claim 4, characterised in that the means comprises means for continuous collection and processing of measurement data and means for presentation of the processed measurement data.
 6. Device according to claim 4, characterised in that means (1) for adjustable application of the measuring head (5, 6) is arranged.
 7. Device according to claim 6, characterised in that the means (1) for adjustable application of the measuring head (5, 6) is arranged between the stand (2) and the measuring head (5, 6). 